This invention relates generally to a power supply circuit which produces a DC output voltage by periodically actuating a switching element and is specifically directed to improving the efficiency of such a switched-mode power supply by increasing the turn-on/turn-off speed of the switching element.
In general, a switched-mode power supply converts a DC input voltage into a DC output voltage which is substantially independent of variations in the input voltage and variations in a load driven by the output voltage. This type of power supply typically includes a series arrangement of an inductance and a controllable switch element connected to a drive or control circuit and responsive to switching pulses generated by the control circuit. The switching pulses alternately drive the switch element into the conducting and the cut-off states for the sequential storage and discharge of energy in the inductance. The aforementioned inductance is typically the primary winding of a transformer which is inductively coupled to a secondary winding thereof. A rectifier couples the output of the secondary winding to a filter capacitor for providing a level DC output voltage to a load.
During the time interval when the switch is cut-off, a substantially large amplitude sinusoidal oscillation is established across the inductance. The frequency of this oscillation is determined by the value of the leakage inductance and of the distributed capacitance in parallel with the inductance. During a portion of the operating cycle of the power supply the switch is rendered conductive causing a current provided by an input voltage source to pass through the inductor so that energy is stored therein. During the other part of the operating cycle, in which the switch nonconductive, the energy stored in the inductor produces a current through the rectifier which charges the capacitor and, consequently, replenishes the energy losses caused by the load. By controlling the conductive period of the switch relative to the total operating cycle, the DC output voltage can be maintained at a constant value despite fluctuations in the input voltage, which typically is derived from an AC line source.
U.S. Pat. No. 3,873,846 to Morio et al discloses a DC power supply circuit of the switching or chopping type which is provided with a switching circuit for an AC power supply and a switching circuit for a DC power supply. The switching circuits are connected to respective primary windings of a transformer having a secondary winding for supplying an operating voltage to a load circuit. Each of the switching circuits includes a switching transistor controlled in its on-duration in accordance with the magnitude or amplitude of the voltage applied to the load circuit so as to maintain the output drive voltage at a predetermined level. U.S. Pat. No. 4,327,404 to Horiguchi discloses a power supply circuit which intermittently produces a DC power source voltage by periodically actuating a switching transistor and smoothing the thus-produced voltage, thereby obtaining a DC voltage of a predetermined level. The output voltage level is compared with a pedetermined reference voltage level so that the ON period of the switching transistor is shortened when the output voltage level is higher than the predetermined reference level, and the ON period of the switching transistor is lengthened when the output voltage level is lower than the predetermined reference level. This approach allegedly provides a stabilized and ripple-free output voltage by controlling the duty cycle of the switching element in a feedback signal arrangement. U.S. Pat. No. 4,276,586 to Boekhorst similarly is directed to a tuned switched-mode power supply for producing a stabilized DC output voltage by controlling the duration of the conducting period of a bidirectional switch as a function of the output voltage. Pulse duration modulation is used to control the drive switching pulses which energize the switching element of this switched-mode power supply. U.S. Pat. No. 4,253,137 to Rao relates to a switched-mode power supply wherein the duration of the conducting state of the switch is adjustable depending upon the output voltage which is the sum of a first DC voltage generated by a first rectifier and a second DC voltage generated by a second rectifier with the first and second DC voltages respectively generated during the conduction and cut-off states of the switching element.
The most common type of switching element used in a switched-mode power supply is a transistor which is driven in an oscillating state by the output of a blocking oscillator. As shown by the aforementioned patents, the duration of the switch-off drive pulses of the switching transistor is generally not constant but varies with the output voltage. If the switch is rendered conductive before inductance oscillation has been completed, or at an instant in which the voltage across the transistor is not zero, large energy dissipation and possible damage may occur in the switching transistor. Protection circuits have been devised in order to avoid the untimely conduction of the switching transistor which typically provide that the ratio between the duration of the switch-off drive pulses of the switching transistor and the period of time between two consecutive cut-off pulses cannot assume certain values.
Another approach to controlling the conduction periods of the switching element in a switched-mode power supply is disclosed in U.S. Pat. No. 4,228,493 to Sartre et al wherein switching element frequency control is accomplished via a delay network by means of a control signal obtained from pulse duration modulation control circuitry, with the duration of the switch-off pulses having a substantially predetermined duration which is longer than half a cycle of the resonant frequency of the resonant circuit comprised of an inductance and a tuning capacitor in the switched-mode power supply. Although providing control over the switching frequency of the switch element with respect to the oscillation of the resonant circuit, this approach requires a relatively complicated pulse duration modulation control circuit in integrated form for generation of the control signals.
These and other problems encountered in the prior art are avoided by the present invention which allows for a rapid switching rate in the switching element of a switched-mode DC power supply in providing for more efficient inductive energy coupling and prolonging the operating lifetime of the semiconductor switching element.